1. Field of the Invention
The present invention relates to a process for fusion-splicing hermetically coated optical fibers each of which has an inorganic coating between an optical fiber glass and an organic coating for protection of the optical fiber.
2. Description of the Related Art
In a certain optical communication field, it is necessary to use optical fibers longer than one kilometer. When such the long optical fiber is used, a technical problem resides in that the fiber does not have a satisfactory mechanical strength. The long optical fiber usually used has a tensile strength in the range of from 50,000 to 80,000 p.s.i. However, in a particular application such as a rapid payment communication system where long optical fibers are used, the optical fiber, as a light waveguide, is required to have the tensile strength more than 200,000 p.s.i.
It is observed that the optical fiber made of silicon oxide material typically has the tensile strength in the order of one million p.s.i., when it is ideally drawn. However, the long optical fiber with such the sufficient mechanical strength cannot be practically produced since there are cracks of submicron scale on the surface of the fiber due to mechanical friction and chemical attack with a contaminant such as steam in an atmosphere during and after usual drawing of the optical fiber. In order to overcome this problem, an organic coating is formed around the optical fiber glass after the drawing thereof.
However, such the organic coating cannot prevent diffusion of steam or hydroxyl ions through the coating, which reduces the strength of the optical fiber coated with the organic material during operation or storage. Then, the optical fiber is very sensitive to steam and other harmful circumstances. Therefore, in order to protect the complete configuration of the optical fiber, a hermetic coating is provided thereon.
When the optical fiber is coated with an inorganic material such as silicone or a metal, one of the most practical methods conventionally applied is Chemical Vapor Deposition (CVD) method. In the CVD method, the coating material is produced in a gas phase through a reaction of single gaseous reactant at a temperature required to produce the coating, or with a reaction of more than two gaseous reactants at a preselected temperature.
However, such the hermetic coating prevents occurrence of the crack due to the contaminant from the outside and dose not improve the strength of the optical fiber. Then, there remains a possibility of breakage of the optical fiber. When the optical fiber breaks, it can be repaired by fusion-splicing. However, the optical fiber glass remains bare in the spliced portion It is clear that the strength of the bare portion is less than that of the other portion in which the hermetic coating is provided
Generally, the strength of the fusion-spliced portion of the optical fiber is less than that of the other portion. For example, in the case of the fusion-splicing with arcing as shown in FIG. 1, it is known that the fused portion (4) has the least strength and that the breakage arises at such the portion. In FIG. 1, (1') indicates an optical fiber, (11) does an optical fiber glass, (2) does an arcing electrode rod and (3) does an arc.
Thus, the fusion-spliced portion of the optical fiber following has four problems:
1. The strength of the optical fiber glass is reduced due to the fusion;
2. With the fusion-spliced portion of the hermetically coated optical fiber, fragments of the hermetic coating material scattered by the arcing remain on the fused portion, which reduces the strength of the spliced portion;
3. No hermetic coating is provided around the fusion-spliced portion; and
4. A side observation method cannot be applied since the hermetic coating prevents the observation.
The problem (1) can be overcome by, for example, re-heating after the fusion-splicing (see Japanese Patent Kokai Publication No. 52011/1980), fusion-splicing in a low moisture atmosphere (see Japanese Patent Kokai Publication No. 220113/1983) or surface treating before the fusion-splicing (see Japanese Patent Kokai Publication No. 42011/1983). Although, in order to overcome the problem (3), a rigid coating (see Japanese Patent Kokai Publication No. 205407/1985) or a metal coating (see Japanese Patent Kokai Publication No. 107007/1985), respectively after the fusion-splicing have been proposed, neither is satisfactory. Further, the problems (2) and (4) have not been overcome.